Method of operation of photoconductive varistor

ABSTRACT

A photoconductive polycrystalline varistor comprising a sintered body of zinc oxide and a minor constituent consisting of other metal oxides or halides has a pair of electrodes on one major face thereof. The electrical resistance between the electrodes is a non-linear function of the electrical potential applied between the electrodes and is additionally a function of the intensity of illumination of the interelectrode gap. The photoconductor sensitivity is further a function of the voltage across the electrodes.

United States Patent [191 Philipp et al.

[ METHOD OF OPERATION OF PHOTOCONDUCTIVE VARISTOR [75] Inventors: Herbert R. Philipp, Scotia; Lionel M. Levinson, Schenectady, both of NY.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Dec. 29, 1972 [21] Appl. No.: 319,346

[52] US. Cl. 250/338 [51] Int. Cl. G0lt 1/16 [58] Field of Search 250/338, 340

[56] References Cited UNITED STATES PATENTS 3,202,820 8/1965 Norton et a1. 250/338 3,453,432 7/1969 McHenry 250/338 Jan. 21, 1975 3,478,210 11/1969 Janacek... 250/338 X 3,610,931 Woolfson 250/338 Primary Examiner-James W. Lawrence Assistant Examiner-Davis L. Willis Attorney, Agent, or Firm--Paul l. Edelson; Joseph T. Cohen; Jerome C. Squillaro [57] ABSTRACT A photoconductive polycrystalline varistor comprising a sintered body of zinc oxide and a minor constituent consisting of other metal oxides or halides has a pair of electrodes on one major face thereof. The electrical resistance between the electrodes is a non-linear function of the electrical potential applied between the electrodes and is additionally a function of the intensity of illumination of the interelectrode gap. The photoconductor sensitivity is further a function of the voltage across the electrodes.

7 Claims, 4 Drawing Figures VOLT/16E 501/6 65 METHOD OF OPERATION OF PHOTOCONDUCTIVE VARISTOR BACKGROUND OF THE INVENTION 1. Scope This invention relates to photoconductors. More particularly, this invention relates to the discovery and embodiments thereof of a photoeonductor effect in polycrystalline varistors comprising sintered bodies of a first metal oxide as a major constituent including additionally an admixture of other metal oxides and/or halides.

2. Prior Art There are a few known substances whose resistance characteristic is non-linear and is expressed by the equation where I is the current flowing through the material,

V is the voltage applied across the material,

C is a constant which is a function of the physical dimensions of the body, its composition, and the parameters of the process employed to form the body, and

a is a constant for a given range of current and is a measure of the non-linearity of the resistance characteristic of the body. v

A well-known example of such varistor materials is silicon carbide. Silicon carbide and other non-metallic I varistor materials are characterized by having an alpha exponent of less than six. Recently, a family of polycrystalline metallic oxide varistor materials have been produced which exhibit an alpha exponent in excess of 10. These new varistor materials comprise a sintered body of zinc oxide crystal grains, including additionally an intergranular layer of other metal oxides and/or halides, as, for example, beryllium oxide, bismuth oxide, bismuth fluoride, or cobalt fluoride, and are described, for example, in U.S. Pat. No. 3,682,841, issued to Matsuoka et al., on Aug. 8, 1972 and U.S. Pat. No. 3,687,871, issued to Masuyama et al., on Aug. 29, 1972.

Substantial effort has been expended in research into the characteristics and applications of this new varistor material, and has resulted in the making of a number of inventions, as for example, those described in U.S.

Pat. Nos. 3,693,053 and 3,694,626 which issued re-' spectively to Thomas E. Anderson on Sept. 19, 1972 and John D. Harnden, Jr. on Sept. 26, 1972 and which are assigned to the assignee of this invention. Until now, however, attempts to produce a photoconductive polycrystalline metal oxide varistor have been unsuccessful.

Photoconductivity of zinc oxide itself is known in the art. Photoconductivity of pure polycrystalline and monocrystalline zinc oxide was experimentally measured and reported by Harrison in 93 Phys. Rev. No. 1 at page 52 in 1954. Furthermore, photoconductivity of zinc oxide and other Group II-Group VI compounds, as for another example, cadmium sulfide, is a theoretically predictable phenomenon.

The photoconductivity of zinc oxide, however, does not contribute photoconductivity to zinc oxide based polycrystalline varistors because the zinc oxide grains are relatively highlyconductive and so any radiation induced changejin resistivity of the zinc oxide grains which may occur is not observable-because of the high resistance of theintergranular material comprising oxides and/or halides ofother metals. Accordingly, any photoconductivity to be found in these polycrystalline varistors must result from photoconductivity ofthe intergranular material. Heretofore, such photoconductivity has not been observed despite experimental efforts to obtain the effect.

CROSS-REFERENCE TO RELATED COPENDING APPLICATION This invention is related to the copending application of Levinson Ser. No. 319,330. filed of even date herewith, assigned to the assignee of this invention, and incorporated herein by reference thereto.

Prior work with polycrystalline metal oxide varistor devices has been directed to devices providing current conduction through the bulk thereof. In the crossreferenced copending application of Levisonit is dis- Another object is toprovide such a device which has simultaneous utility a photodetector and as a varistor.

Briefly, and in accordance with one embodiment of this invention, there is provided a photoconductive polycrystalline metal oxide varistor in the form ofa sintered pellet or disk having a pair of electrodesdisposed= on one surface thereof. The electrodes have a gap therebetween which provides for the conduction of electrical currents from one electrode to another through a portion of the varistor material and provides for illumination of the current-carrying portion ofthe I varistor material. Thepotential applied across the electrodes is selected to provide the desired photoconductor sensitivity and photocurrent magnitude. I

The-novel features of this invention sought to be patented are set forth with particularity in the appended claims. The invention, together with further object and advantages thereof, may be understood from a reading of the following specification and appended claims in view of the accompanying drawing in which":

FIG. 1 is a plan view of a photoconductive varistor which may be used in practicing this invention.

FIG. 2 is a plan view'of a modification of the photoconductive varistor of FIG. I having four electrodes disposed on a face thereof, and being adapted for use in a photodetector circuit including a dark current bucking circuit in accordance with this invention.

FIG. 3 is a plan view of a modification of the photoconductive varistor of FIG. 1 having coaxially configured electrodes.

FIG. 4 is an elevation view of a photoconductive varistor in accordance with one embodiment of this invention wherein the photoconductive varistor of FIG. 1 is provided with electrodes on both major faces thereof, whereby one pair of electrodes is shielded from energy incident on the other pair of electrodes by rial having a surface on which are disposed first and second electrodes 11 and 12. Electrodes I1 and 12 have respectively substantially parallel facing edges 13 and 14 which cooperatively define a gap 15 between electrodes 11 and 12 on the surface of polycrystalline varistor l0.

Varistor 10 is shown in the drawing as having a circular shaped surface and represents a disk or plinth of polycrystalline varistor material. The disk or plinth shape is a convenient one to manufacture, but this invention is not so limited and other shapes, as for example, a rectangular prism shape may be employed if desired for design reasons.

Polycrystalline varistor disk 10 is fabricated by pressing and sintering a mixture comprising zinc oxide as a major constituent and other metal oxides and/or halides as minor constituents. For a more detailed general description of the composition and method of fabrication of the polycrystalline varistor disk, reference is made to the aforementioned patents of Matsuoka et al., Masuyama et al., Anderson, and Harnden, Jr. In one embodiment of this invention the varistor disk is comprised by weight of principally zinc oxide with bismuth oxide and antimony oxide as secondary constituents and smaller amounts of each of other metal oxides. This embodiment exhibited a varistor alpha exponent of 40.

Electrodes l1 and 12 are applied to a face of polycrystalline varistor disk 10 in a convenient manner known in the art and may, for example, be applied as silver paint or as evaporated or sputtered metals such as Aluminum, Zinc, or Platinum. The spacing between parallel facing edges 13 and 14 of the electrodes is selected to determine the varistor voltage of the device. The varistor voltage is accordingly a function of the width of gap 15. Gap 15 also provides for illumination of the current-carrying portion of varistor disk 10.

In operation, the application of an electrical potential difference between electrodes 11 and 12 causes a current to flow between the electrodes 11 and 12 through portion 15 of varistor disk 10. The magnitude of this current is given by the equation:

where B is a positive function of the intensity and frequency of electromagnetic energy illuminating gap 15, and of the electrical potential across gap 15; the other parameters being identified above.

As indicated by the equation immediately above, the magnitude of the photocurrent generated in a photoconductive polycrystalline varistor in accordance with this invention is a function of the electrical potential between the electrodes. The photosensitivity of the photoconductive varistor is also a function of the potential across the gap. Sensitivity is inversely proportional to the potential; that is, at lower voltages the ratio of dark current plus photocurrent to dark current is greater than it is at higher voltages. The following table illustrates the relationship between the electrical potential across gap 15 on the one hand and photocurrent magnitude and photoconductor sensitivity on the other hand.

As the above table indicates, the photoconductive varistor of this invention is extremely sensitive at low operating voltages. For example, in the range 0.5 to 5 volts, the ratio of photocurrent plus dark current to dark current is on the order of 6,000. On the other hand, in this low voltage range, the photocurrent magnitude is quite small. In accordance with this invention, therefore, operation of the photoconductive varistor at low voltages is useful in applications such as precision radiation intensity measurements in which the highest sensitivity is required and in which high gain, low noise, amplifiers may be conveniently incorporated in the utilization apparatus which receives the output ofthe photoconductive varistor.

The table also indicates that at high applied voltages a relatively large photocurrent magnitude is obtained.

On the other hand, the sensitivity of the photoconductive varistor is greatly decreased. For example. the table indicates that at 128 volts across gap 15 the ratio of photocurrent plus dark current to dark current is two. One milliampere of photocurrent, however, is

available and this compares favorably with photomultiplier tubes operating in the current mode, for example, and is substantially less expensive to provide by this inventive technique. At this point, it should be noted that the data presented in Table I encompass the voltage range 0.5 to 128 volts applied across the interelectrode gap of a particular photoconductive polycrystalline varistor specimen. The data presented were taken in a carefully performed and controlled experiment, but this invention is not to be considered to be limited by the data of Table I, since other experiments have indicated that the data of Table I can be safely extrapolated with accuracy. In accordance with this invention, operation of a photoconductive polycrystalline varistor at high voltages is useful in applications in which direct utilization of the photocurrent output of a photodetector is desired, as, for example, to operate a small motor, for automatic shutter adjustment, for example.

As may be further recognized from Table I, high voltage operation of the photoconductive polycrystalline varistor of this invention results in rather large dark currents which may be quite inconvenient in some applications. Accordingly, in such applications, compensation for dark current is provided in accordance with this invention. At a given operating voltage, the dark current is a constant and so compensation may be provided by bucking the dark current in a bridge circuit when a constant current source comprising a battery and a resistor as is known in the art. Another prior art compensation method which may be employed with the photoconductive varistor of this invention is to '5 chop the energy incident on gap 15 and to employ a frequency discriminatory utilization device as, for example, an amplifier circuit having series capacitor input means. Alternatively, compensation may be provided in accordance with the embodiment of this invention illustrated in FIG. 2. In the FIG. 2 embodiment varistor disk has two pairs of electrodes disposed on a face thereof. Electrodes 11a and 12a comprise the photoconductor electrodes and operate as described above with reference to electrodes 11 and 12 of FIG. 1. Electrodes 11b and 12b are the reference electrodes. The portion of gap 15 comprising the varistor conductive path between electrodes 11b and 12b is covered with a shield member not shown for preventing radiation from impinging thereon. Accordingly, dark current plus photocurrent flows between electrodes 11a and 12a and dark current only flows between electrodes 11b and 12b. Because of the nature of the varistor material, cross currents, as, for example, between electrodes 11b and 12a, or 1111 and 12 b, will not flow. The current flowing between electrodes 11b and 12b is then bucked against the current flowing between electrodes 11a and 12a in a conventional bridge circuit to eliminate the dark current component from the output of the photoconductive varistor. This method has the advantage over the constant current source bucking method of being usable, for a given photoconductive varistor'device, at any operating voltage without need for changing components of a bucking current source. This method has the advantage over the prior art chopping method of providing a DC. output signal at a higher power level for utilization.

FIG. 3 illustrates an alternative embodiment of the photoconductive polycrystalline varistor of this invention having coaxially disposed electrodes and comprises varistor disk 10 having circular electrode 120 and annular electrode llc disposed thereon and defining annular gap 15 therebetween.

The compensation method described with reference to FIG. 2 may be employed with a modification of electrode arrangement and consequent further simplification.- In the embodiments illustrated in FIGS. 1 and 3, a second pair of electrodes in the same pattern as those shown is disposed on the opposite, unseen, face of varistor disk 10, as illustrated in FIG. 4. One such set of electrodes 11 and 12 as shown in FIG. 4 functions as the photoconductor electrode pair and the other 21 understood that within the scope of the appended and 22 as shown in FIG. 4 serves to provide the referclaims the invention may be practiced otherwise than is specifically described.

The invention claimed is:

l. A method of operating a photoconductive varistor having a first pair of electrodes disposed on a face of a body of polycrystalline varistor material. said electrodes having a gap therebetween, said method comprising the steps of:

applyling an electrical potential between said electrodes, said potential being preselected to determine the sensitivity and magnitude of photocurrent of said photoconductive varistor;

positioning said photoconductive varistor to receive energy to be detected; and

receiving, in a utilization device, an electrical current flowing between said electrodes across said gap, said current being proportional to said potential and to the intensity of said energy;

the ratio of said photocurrent plus dark current to dark current being inversely proportional to'said potential. 2. The method of claim 1 wherein maximum sensitivity is desired and said electrical potential is preselected at a low voltage level.

3.'The method of claim 1 wherein a large photocurrent magnitude is desired and said electrical potential is preselected at a high voltage level.

4. The method of claim 1 wherein a dark current flows between said electrodes in the absence of energy 6. The method of claim 4 wherein a second pair of electrodes is disposed .on a face of said body, said second pair of electrodes having a second gap therebe tween, said second gap being shielded from radiated energy so that only dark current flows between said second pair of electrodes and wherein said eliminating step comprises:

bucking, in a bridge circuit, said current flowing between said first pair of electrodes against said dark current flowing between said second pair of electrodes.

7. The method of claim 4 wherein said eliminating step includes chopping said energy to be detected and said receiving step includes rejecting a DC. component from said current proportional to said intensity of said energy. 

1. A method of operating a photoconductive varistor having a first pair of electrodes disposed on a face of a body of polycrystalline varistor material, said electrodes having a gap therebetween, said method comprising the steps of: applyling an electrical potential between said electrodes, said potential being preselected to determine the sensitivity and magnitude of photocurrent of said photoconductive varistor; positioning said photoconductive varistor to receive energy to be detected; and receiving, in a utilization device, an electrical current flowing between said electrodes across said gap, said current being proportional to said potential and to the intensity of said energy; the ratio of said photocurrent plus dark current to dark current being inversely proportional to said potential.
 2. The method of claim 1 wherein maximum sensitivity is desired and said electrical potential is preselected at a low voltage level.
 3. The method of claim 1 wherein a large photocurrent magnitude is desired and said electrical potential is preselected at a high voltage level.
 4. The method of claim 1 wherein a dark current flows between said electrodes in the absence of energy received by said photoconductive varistor and further comprising the step of: eliminating said dark current from said electrical current proportional to said intensity of said energy.
 5. The method of claim 4 wherein said eliminating step comprises: bucking, in a bridge circuit, said current flowing between said first pair of electrodes, against a constant current equal to said dark current.
 6. The method of claim 4 wherein a second pair of electrodes is disposed on a face of said body, said second pair of electrodes having a second gap therebetween, said second gap being shielded from radiated energy so that only dark current flows between said second pair of electrodes and wherein said eliminating step comprises: bucking, in a bridge circuit, said current flowing between said first pair of electrodes against said dark current flowing between said second pair of electrodes.
 7. The method of claim 4 wherein said eliminating step includes chopping said energy to be detected and said receiving step includes rejecting a D.C. component from said current proportional to said intensity of said energy. 